The present invention relates generally to wireless communications systems, and more specifically to adaptive modulation techniques for wireless broadband communications systems that provide for increased data rates while taking into account channel interference resulting from operating in shared frequency bands, signal fading resulting from dynamic channel degradation, and signal distortion resulting from compliance with maximum power output regulations.
In recent years, there has been an increasing need for wireless communications systems capable of carrying broadband data. Such a need has arisen for a variety of reasons, including the difficulties that users often experience in obtaining high speed Internet access service in some remote areas, and the convenience that wireless systems generally afford to users wishing to set up communications links and networks accessible from locations within urban environments or from locations dispersed across significant geographical distances. A conventional wireless broadband communications system can be configured as a line-of-sight (LOS) system or a non-line-of-sight (NLOS) system. The conventional LOS system typically includes a directional antenna deployed at one or more user locations within the line-of-sight of an antenna at a base station. The conventional NLOS system is typically configured as a multiple input multiple output (MIMO) system including a first plurality of antennas deployed at one end of a communications link, and a second plurality of antennas deployed at the other end of the communications link. Both the LOS system and the NLOS system may be employed in point-to-point and point-to-multipoint applications, in which a transmitter transmits signals over multiple communications channels using known space-time coding and modulation techniques, and one or more receivers capture the transmitted signals and employ signal processing techniques to decode and demodulate the signals to recover user data.
One problem facing conventional wireless broadband communications systems, whether they are configured as LOS or NLOS systems, is the need to maintain high data rates during the provision of multi-media services involving text, graphics, video, animation, and/or sound. Several aspects of the environment in which wireless communications systems typically operate have exacerbated this problem, such as the limited bandwidth capacity of the channels available to the systems. In a wireless communications system, communications channels are typically defined within a limited portion of the electromagnetic spectrum allocated to the system. To achieve more efficient use of the allocated spectrum and higher data rates, wireless communications systems have employed digital communications techniques that allow data packets corresponding to separate communications sessions to be transmitted along multiple shared channels, obviating the need for a single dedicated channel for each communications session. Such techniques are frequently employed in wireless communications networks including one or more wireless LANs (WLANs), which utilize data packet protocols to communicate between the various nodes of the WLAN. The operational parameters for such WLANs are described in the IEEE 802.11 standard.
More recently, wireless communications systems have begun operating in shared frequency bands such as the unlicensed frequency band for WLANs located at 5 GHz. Due to the unlicensed nature of the shared 5 GHz band, more than one WLAN operating in the 5 GHz band may be deployed within the same geographical area. Further, other types of wireless communications systems and radar operating in the 5 GHz band, or generating frequency harmonics having components in the 5 GHz band, may also be deployed within the same geographical area. As a result, the levels of noise and interference on the channels available to wireless communications systems are likely to increase, thereby hindering the systems' ability to maintain high data rates appropriate for all multi-media services.
Another aspect of the environment affecting the performance of wireless broadband communications systems, particularly, wireless communications systems operating at frequencies above several GHz, is time variant channel degradation resulting from changing environmental conditions caused by, e.g., precipitation and the growth and movement of trees and other foliage. For example, radio frequency signals propagating through rain and windy tree environments can undergo significant signal attenuation and fading. Conventional wireless communications systems have compensated for such dynamic channel degradation by employing robust modulation schemes such as QPSK (quadrature phase shift keying), which typically delivers relatively low bit error rates in poor environmental conditions. However, conventional systems employing such robust modulation schemes have generally failed to provide high data transmission rates.
Conventional wireless broadband communications systems have also implemented adaptive modulation techniques to adjust transmission parameters such as a modulation level and a coding rate to accommodate for changes in channel characteristics affecting the rate of data transmission. However, in general, the variability of environmental conditions caused by the wind speed, and the movement and location of objects relative to the signal path and their effect on signal fading have not been satisfactorily addressed in such conventional systems. In addition, conventional wireless communications systems implementing adaptive modulation techniques have generally failed to take into account the signal distortion that can result from compliance with maximum power output regulations, e.g., FCC regulations limiting the maximum power output to 1 watt.
It would therefore be desirable to have a wireless broadband communications system capable of maintaining high data rates appropriate for multi-media services, while avoiding the drawbacks of the above-described conventional wireless communications systems.